1. Field of the Invention
The present invention relates generally to a compressor which compresses fluid, introduced into cylinder bores, by reciprocating pistons. More particularly, it pertains to a wave cam plate type compressor which reciprocates pistons by rotating a wave cam integrally attached to a drive shaft.
2. Description of the Related Art
Prior art wave cam type compressors are provided with a drive shaft, a wave cam, and pistons accommodated in associated cylinder bores. The wave cam is integrally fixed to the drive shaft and connected to each piston. In this type of compressor, fluid introduced into the cylinder bores is compressed by reciprocating movement of the pistons within the bores. The reciprocation is caused by integral rotation of the drive shaft and the wave cam. Japanese Unexamined Patent Publication No. 57-110783 and Japanese Unexamined Utility Model Publication No. 63-147571 disclose such compressors.
In the compressor of the Japanese Unexamined Patent Publication No. 57-110783, a wave cam, having a front and a rear surface, and double-headed pistons is disclosed. A roller, interposed between each cam surface and each piston, is rotatably and permanently fitted to each piston. Rotation of the wave cam rolls the rollers with respect to the wave cam surfaces thus axially displacing the contact point between the roller and the piston to reciprocate the pistons. The reciprocation of the pistons is based on a displacement curve of the cam surface. In the compressor of Japanese Unexamined Utility Model publication No. 63-147571, a wave cam, having a cam groove respectively defined on its front and rear surfaces, and double-headed pistons are employed. A ball is interposed between each cam groove and each piston. This compressor differs from the former compressor in that balls are utilized instead of rollers and that the reciprocation of the pistons is based on a displacement curve of the cam groove during rotation of the wave cam.
In a swash plate type compressor, which employs a swash plate in place of the wave cam, the swash plate is rotated by a drive shaft to reciprocate pistons accommodated in cylinder bores for compression of fluid supplied to the bores. In this compressor, periodic displacement of a point following the swash plate surface during one rotation of the swash plate is represented by a single cycle sine wave curve. Hence, one compression stroke is performed per rotation of the drive shaft in the swash plate type compressor. On the other hand, in a wave cam type compressor, periodic displacement of a point following the wave cam surface during one rotation of the wave cam shows a plurality of sine wave curve cycles. Hence, a plurality of compression strokes are performed per rotation of the drive shaft in the wave cam type compressor. Thus a wave type cam compressor has a larger compressing volume per rotation then a swash plate type compressor.
In a wave cam type compressor, the roller or the ball is interposed between each cam surface and each associated piston. Each roller or ball rolls with respect to the wave cam. Linear contact takes place between the outer surface of the rollers and the cam surface when the rollers roll. Point contact takes place between the outer surface of the ball and the cam surface when the ball rolls. However, a microscopic view of the portions which contact the cam surface on the outer surface of the roller or ball reveals resilient deformation caused by the pressure (hereinafter referred to as "contact pressure") applied to the roller or ball during contact with the cam surface as the roller or ball rolls thereon. That is, the resilient deformation of the cam surface caused by the roller or ball leads to planar contact between the outer surface of the roller or ball and the resiliently deformed cam surface of the wave cam. Accordingly, it is preferable that the contact pressure be low. Low contact pressure is an important factor for improving the durability of the compressor. The longer the length of linear contact between the outer surface of the roller and the can surface of the wave cam is, or the smaller the curvature of the rollers of balls is (i.e., the larger the radius of curvature of the roller or ball), the smaller the contact pressure is. In other words, a small curvature of the rollers or balls, which allows more surface area of the roller or the ball to come into contact with the cam surface, reduces the contact pressure.
When a roller is interposed between the cam surface of the wave cam and the piston, contact pressure can be reduced by lengthening the roller along its axial direction or enlarging the diameter of the roller. When a ball is interposed between the cam surface of the wave cam and the piston, contact pressure can be reduced by enlarging the diameter of the ball. However, a roller or ball is retained in a recess defined in the double-headed piston of the wave cam type compressor. Therefore, the length of the roller, the diameter of the roller, and the ball diameter are limited in relation to the diameter of the piston. In other words, it is necessary for the diameter of the piston to be enlarged to make the length and diameter of the roller or ball diameter larger. A piston with a larger diameter will lead to enlargement of the compressor.